A plunger lift is an artificial lift method that is used to remove fluids from a gas well. A plunger lift system uses a freely moving plunger in the production tubing. A seal is formed between the plunger and the production tubing that prevents fluid from passing between the plunger and the wall of the production tubing. The plunger is allowed to sit at the bottom of the well until sufficient pressure builds up behind the plunger and then the plunger is allowed to rise to the top of the well. Fluid that has accumulated on top of the plunger is carried up the well by the plunger to the well head, where this fluid is then removed from the well.
The movement of the plunger is controlled by opening and closing a valve between the production tubing and an outlet line (commonly called a sales line). When the valve is closed, the plunger drops to the bottom of the well. With the valve closed, the pressure from the well builds up and when a desired pressure level is reached, the valve can be opened, connecting the production tubing with the outlet line. Because the outline line is typically at a lower pressure than the elevated pressure in the production tubing, the gas in the production tubing flows out of the well through the open valve and into the outlet line. This causes the plunger to rise in the well. When the plunger rises into the well head, it can then be held in the well head until the gas exiting the production well through the open valve is sufficiently reduced and the plunger can then fall back down the production tubing.
The time the plunger is held in the well head and the valve is left open is called the “afterflow” time. This afterflow time is the time that gas is being produced from the well by allowing it to leave the well and enter the outlet line. However, having too large of an afterflow time can cause too much water to enter the well casing causing the well to “water in”. This can occur when the buildup of water in the well causes a hydrostatic barrier preventing gas from the formation from exiting the well. Over time, as more and more water is removed from the well casing by the plunger, the afterflow time may be able to be lengthened.
Typically, electronic controllers are used to control the operation of the plunger lift system. The electronic controller is used to control the opening and closing of the valve based on an afterflow time and a close time. Typically, these plunger lift systems will have a plunger arrival sensor positioned near the top of the well (usually in a plunger receiver in the well head) that can sense when the plunger has reached the top of the well.
In these systems, the controller can typically determine an average velocity of the plunger during its trip up the well. It can do this by determining the time when the valve was first opened, causing the plunger to begin to rise, and the time the plunger arrival sensor determines that the plunger has reached the top of the well. With this trip time and the depth of the well, the controller can calculate an average velocity that the plunger was travelling during its trip to the top of the well.
However, this calculated average velocity is just that; an average velocity of the plunger over the entire trip up the well. It does not take into account the fact that the plunger may travel at different speeds as it travels up the well. For example, the plunger could be traveling much slower at the bottom of the well because it is just starting to move and will pick up speed as it continues to rise up the well. Additionally, the plunger may be picking up speed throughout its entire trip up the well and may be travelling faster at the top of the well than the average velocity. This acceleration of the plunger could be due to a number of factors, such as the loss of fluid from above the plunger, decompressing of the gas, a hole in the tubing, fluids unloading above the plunger down the sales line, etc. The use of an average velocity of the plunger during its trip up the well does not allow the controller to know how fast the plunger is moving at the top of the well.
Not knowing the velocity of the plunger can create a number of problems. First, if the plunger is moving too fast when it enters the well head it can damage the well head. Typically, the plunger receiver that stops the plunger when it reaches the well head contains a spring, rubber damper, etc. that the plunger can impact against when it reaches the plunger receiver in order to stop the plunger. However, if the plunger is travelling too fast when it reaches the top of the well and enters the plunger receiver, it can hit the top of the plunger receiver too hard and damage the plunger receiver and the well head. The speed the well head can handle will depend on the well head and the weight of the plunger. Currently, when the average velocity of the plunger is used, it cannot be accurately determined what the velocity of the plunger is when it reaches the plunger receiver. It could be travelling slower than the average velocity, or conversely, it could be traveling faster than the average velocity.
The controller typically uses a threshold velocity as a warning that the plunger is travelling too fast and can damage the well head. If the average velocity approaches or surpasses the threshold velocity, the controller knows that damage to the well head may occur and can act accordingly (such as shutting down the plunger lift system). However, because the average velocity is not a clear indicator of the velocity of the plunger as it reaches the top of the well, typically the controllers use a threshold velocity that is lower than what they could use to take into account the fact that the average velocity determined by the controller during a trip up the well by the plunger may be lower than the speed of the plunger at the top of the well. This can cause the velocity of the plunger to be set by the controller to a lower value than may be ideal to try and ensure that the plunger is not traveling too fast at the top of the well. Alternatively, if the plunger is slowing near the top of the well because of the pressure of the well above the plunger, the speed of the plunger at the top of the well may be lower than the average velocity. With the controller using a lower threshold for the velocity than is required, this could mean the plunger could be allowed to travel faster than the threshold value because the plunger is actually slowing near the top of the well.
The use of the average velocity of the plunger on its trip up the well can also cause inefficiencies when the average velocity or trip time is being used to try and optimize the operation of the plunger lift system. Typically, the theory behind the operation of a plunger lift system is to try and have the plunger rising at a velocity that is not too slow to cause water and/or other fluids being carried above the plunger to fall off the top of the plunger, but not rising so fast that it breaks components and causes damage to the well head when it reaches the top of the well. However, because an average velocity is typically used by a controller of the plunger lift system to control the operation of the plunger lift system, to account for the fact that the velocity could be higher than the average velocity when the plunger reaches the top of the well, the average velocity that these systems try to have the plunger rise at is typically lower than it could ideally be. By using the average velocity, these systems tend to be a bit less efficient than they could be because they tend to build in a margin to account for the fact that the average velocity of the plunger is being used rather than a velocity of the plunger at the top of the well.